Liquid crystal displays (LCDs) are widely used in electronic display devices, such as computer monitors, handheld devices and televisions. Unlike cathode ray tube (CRT) displays, LCDs do not emit light and, thus, require a separate light source for viewing images formed on such displays. Ambient light illumination is sufficient for some applications, but with most large area and high performance LCDs, ambient light causes glare and is detrimental to readability. Thus, in order to improve readability, most large area and high performance LCDs include a source of light located behind the display, which is usually referred to as a “backlight.”
Presently, many popular systems for backlighting LCDs include direct-lit backlights, in which multiple lamps or a single serpentine-shaped lamp are arranged behind the display in the field of view of the user, or edge-lit backlights, in which the lamps are placed along one or more edges of a lightguide located behind the display, so that the lamps are out of the field of view of the user. In order to compete with CRT displays, large LCD displays (e.g., greater than ˜20″ or 50 cm in diagonal) must have high luminance targets, e.g., about 500 nt or more. Such high luminance targets are currently met by single-cavity direct-lit backlights for LCDs.
The use of conventional direct-lit backlighting systems, however, has caused some concerns among manufacturers of large LCDs, such as LCD televisions. One concern is a discrepancy between the intended lifetimes of LCD televisions, which for most consumers may be 10 to 20 years, and the lifetimes of individual lamps in the televisions' backlights, which are approximately 10,000 to 20,000 hours and usually at the lower end of this range. In particular, cold cathode fluorescent lamps (CCFLs), which are frequently used for backlighting, have varying lifetimes and aging characteristics. If one CCFL burns out in a conventional direct-lit backlight, the result will be a dark line directly across the display. In addition, the spatial color uniformity of a direct-lit display suffers as each CCFL ages differently. Major LCD manufacturers and television set makers currently do not have a model for servicing LCD television backlights that fail in either of these two modes.
Furthermore, light reaching the viewer from multiple sources in a conventional direct-lit backlight usually is not mixed as well as the light in edge-lit backlights. Nonetheless, despite this and other shortcomings, as well as the uniformity and aging disadvantages of conventional direct-lit backlights, they are currently a popular choice for backlighting LCD televisions, because they allow reaching luminance targets that are competitive with CRT televisions. On the other hand, although edge-lit-backlights would appear to be more advantageous in many respects, achieving desired levels of luminance with conventional edge-lit backlights has remained a challenge. One difficulty has been arranging a large enough number of light sources at an edge of a single lightguide to provide sufficient optical power to reach the target luminance. Other difficulties include enhancement film warping in conventional backlights, e.g., due to high thermal gradients and handling problems.
Thus, there remains a need in the field of backlights for large high-performance LCDs for backlighting systems that are capable of achieving high luminance targets and are more efficient. In addition, there remains a need for backlighting systems for large high-performance LCDs that overcome other shortcomings of the currently available backlighting systems described above.